A typical X-ray source comprises a thermionic source (typically a heated filament), a high-voltage supply to accelerate the electrons to a high energy, and a target made of a high atomic number metal.
FIG. 1 depicts a simple schematic diagram of a very basic and conventional X-ray source, although it will be realised that, in practice, much more complex arrangements are generally used, including the use of additional electrodes and magnetic fields to control and focus the electron beam
Electrons are emitted thermionically from a hot cathode filament 30 under the action of an isolated heater supply 10 and are attracted to a metal target 70 via an intervening anode 60. The electrons are accelerated in a beam 50 towards the target due to a high potential difference between the filament and the anode/target arrangement established by means of a high voltage supply 20. On striking the target 70 the electrons stimulate X-ray emission by various processes, resulting in the emission of an X-ray beam 80.
Since it is desirable for the anode and target to be at, or substantially near, ground potential, the cathode filament must be at a very high negative potential with respect to ground Moreover, the cathode filament requires several watts of power to reach operable temperatures.
FIG. 2 shows a typical X-ray source arrangement where a cathode filament 30 is heated by a voltage supplied from an isolating transformer 11. Typically the voltage is between 2V and 6V, whilst the electrons are accelerated by a high voltage supplied from a multiplier 90, known as a Cockcroft-Walton voltage multiplier. The high voltage maybe in the range of hundreds of kilovolts, for example 160 kV.
It is often required to construct an X-ray source that is compact, and this requirement introduces or exacerbates various problems, for example those associated with providing accurate and effective control over the electron beam current, particularly where the source is desired to be capable of operating reliably with a low radiation output, and those associated with achieving sufficient insulation between various components.
Control over the current of the electron beam 50 is usually desirable with X-ray sources in general and, in low performance X-ray sources, this is frequently achieved merely by varying the temperature of the filament; relying upon the principle that a hotter filament emits more current than does a cooler one. In higher performance systems, exemplified in very basic form in FIG. 3, this is achieved by controlling the beam in the space charge limited regime by means of a field control electrode 40, often referred to as a focusing cup or Wehnelt. Such a focusing cup 40 is required to be at a negative potential with respect to the cathode filament in much the same way as the grid in a thermionic triode valve. The required potential can be supplied by either an electrically isolated bias supply, or self-biasing using a feedback resistor 120 between cathode filament 30 and focus cup 40. Current passing through the feedback resistor generates the required negative bias. However, such a negative feedback system has the drawback that it is difficult to adjust.
When conventional X-ray sources are required to operate at low electron beam current levels, a problem occurs in that electron current leakage from the cathode and focus cup becomes significant compared to the total electron beam current. Often this problem arises from cold cathode discharge (field emission), ‘surface tracking’ or other such problematic phenomena. Conventional X-ray sources measure the electron beam current with a current sensing circuit located at the end of the high voltage supply that is at ground potential (shown schematically as 25 in FIG. 4). A problem then arises in that any current measurement at this point in the system cannot differentiate between the actual thermionic electron beam current and the leakage current. This inability to
separate the level of current leakage from the overall current measurement leads to variations in X-ray output since accurate control over the true electron beam current is not possible. Particularly where low radiation output levels are called for, variations in the measured electron beam current due to spurious factors such as those mentioned above can have a significant and adverse effect upon the radiation output levels and stability of operation.
Another problem with conventional X-ray sources arises from the high voltages required to accelerate the electron beam. When employing such extreme potential differences, there is always a risk of an electrical discharge or breakdown. When such phenomena occur, rapidly changing electromagnetic fields arise. Such fields induce large currents to instantaneously flow within the electronic circuitry of the X-ray source, and these currents can damage or destroy circuit components leading to X-ray source failure. A common solution to this problem is to enclose all susceptible components and circuitry within a Faraday shield to protect them from any rapidly changing fields.
In known X-ray sources, the integrity of the Faraday shield is compromised by the need to leave a conduit through which power and signals can be introduced into the circuitry. The break in the shield to provide a signal path also provides a pathway for signal interference during a high voltage breakdown. The integrity of the shield is particularly compromised by the use of isolating transformers that are generally used to introduce power and signals into the Faraday shield.
The present invention arose in an attempt to address some or all of the above problems.